A Closer Look: Why do We Need Three Scales of Temperature?

Thermometer showing the Fahrenheit
and
Celsius scales.

As mentioned in the video, the two scales we are most familiar with, the
Fahrenheit and Celsius (also known as centigrade) scales, are defined in
terms of events that are universal: a temperature of zero on the Fahrenheit
scale is the temperature of a mixture of equal parts ice, water, and salt,
and the freezing point of water is what sets the zero point on the Celsius
scale. A difference of one degree Celsius is larger than a difference of
one degree Fahrenheit.

So why do we need yet another temperature scale?

If we examine the definition
of temperature as it relates to our particle model, we can see that there
is something more fundamental on which to
base our temperature scale. Temperature is related to the average energy
of the motion of the particles of the object. Therefore, a natural
point for a temperature scale is the point at which all particle motion
stops.
This point is defined as zero on the Kelvin scale. The unit of the
Kelvin scale is referred to as a "Kelvin," and the magnitude
of one Kelvin is the same as the magnitude of one degree Celsius. The zero
temperature
on the Kelvin scale is called absolute zero.

Is it possible to reach
absolute zero?

Thermometer using the Kelvin
scale.

In a word, no. As we've seen in previous sessions, successful
scientific models often break down when they are applied to circumstances
with extreme conditions: for example, mass conservation breaks down
when we deal with
the high temperatures and pressures inside a star. Very low temperatures
are another extreme. As it turns out, the best model for understanding
the world of the very small, quantum mechanics, dictates that
we cannot completely stop the motion of any particles, no matter how
hard we
try. However, with certain special techniques (including using
the
light from
lasers to slow down particles), scientists have been able to
lower the temperature of matter to just a fraction of a Kelvin above
absolute zero.

At these very low temperatures, matter behaves very differently
than it does near room temperature. As a result, scientists
had to develop
further
refinements to their particle model. However, since everyday
life is experienced between the extremes of high and low temperatures,
we do
not observe or
experience any of these unusual effects.